It's the secret entrance to the underground tunnel the cartel uses to move stuff back and forth to Baja.
uff:
Yes, I'm totally kidding. I've seen it, but have no idea what its real purpose is/was. Sort of like across the channel there on La Pinta. When I first went there years ago, everybody called it Nacho Beach because Nacho lived there in the old Gillispie house. We'd go up to the house to find him and pay for our stay on the beach, or get him to take us out in a panga to fish the channel. For years, I had no idea it was really named La Pinta, just Nacho Beach.